The referenced U.S. Pat. No. 4,126,790 relates to a smoke detector, a salient feature of which is pulsed or strobed operation in which the detection circuitry, with or without the voltage regulator, is operated periodically at a relatively low duty cycle. Energy storage elements are preferably incorporated to build up and hold an electrical charge between operating times. In this manner, power consumption is significantly reduced as is peak or transient current demand.
The referenced U.S. Pat. No. 4,121,110 relates to the optical biasing of an optical smoke detector and to various physical features of the biasing system and its housing which facilitate the passage of smoke into the detection chamber. Various adjustable arrangements of radiant energy source and transducer disclosed therein enhance the effectiveness of the optical biasing and further increase the efficiency of smoke detection.
Thus the goals of lower power consumption and lower transient current demand are met by the strobed smoke detectors described in U.S. Pat. No. 4,126,790 and the goals of greater sensitivity and stability of operation are met by the optically biased photo-optical smoke detectors of U.S. Pat. No. 4,121,110.
As is pointed out in the patent application and referenced patents, the basis of operation of most smoke detectors, either photo-optical or ionization, is the effect of smoke upon the amount of energy received by a transducer from a known energy source. Most commonly, in optical detectors, it is the presence of smoke in a detection chamber which reflects or scatters light not normally incident upon the receiving transducer into the field of view of that receiver, thereby changing the electrical output of the receiving transducer to trigger an alarm. In order to reduce the effects of outside ambient energy, the detection chamber is made "light-tight" whereby smoke, but not light, may enter the chamber by a tortuous path. It would clearly be better if a photo-optical smoke detector could operate with an essentially open structure, relatively immune to ambient energy, into which smoke could flow entirely freely. This would increase the speed at which smoke is detected. It would further be desirable to increase the energy of the source so as to increase the signal-to-noise ratio in order to provide a more reliable, noise-free signal to the alarm circuitry.
While ambient energy in the detection chamber is primarily a problem solely with photo-optical detectors, other problems affecting the sensitivity and maintenance of the detector are common to all smoke detectors. Thus, a self-compensating smoke detector in which sensitivity would be automatically adjusted to remain substantially constant despite aging of components, accumulation of dust particles on exposed elements, drift of components and the like would greatly reduce maintenance efforts and costs and would increase reliability. There are also situations in which a rate-of-rise detection would be useful to advantageously distinguish between slow and rapid changes of smoke density. Finally, constant automatic supervision of the integrity and operability of a smoke detector is of obvious importance as in any safety device. It is with these desiderata that this invention is concerned.
A principle object of the invention is an improved smoke detector into which smoke may flow more freely. Another object is to compensate for ambient energy which may reach the detection chamber. A further object of the invention is to adjust automatically the gain of the smoke detection circuitry to compensate for changes in operating conditions, for example, due to aging or drift, of the components.
A still further object of the invention is a reliable and inexpensive smoke detector which operates with exceedingly low power drain, and in which maintenance is minimal. A further object of the invention is a smoke detector in which available radiant energy within the detection chamber of the apparatus is increased.
Other objects of the invention are a smoke detector responsive to the rate of increase in the smoke level, and a smoke detector having an increased signal-to-noise ratio.